Khândogya Upanishad: VI, 12

TWELFTH KHANDA 1. 1. 'Fetch me from thence a fruit of the Nyagrodha tree.' 'Here is one, Sir.' Break it.' 'It is broken, Sir.' 'What do you see there?' 'These seeds, almost infinitesimal.' 'Break one of them.' 'It is broken, Sir.' 'What do you see there?' 'Not anything, Sir.' 2. The father said: 'My son, that subtile essence which you do not perceive there, of that very essence this great Nyagrodha tree exists. 3. 'Believe it, my son. That which is the subtile essence, in it all that exists has its self. It is the True. It is the Self, and thou, O Svetaketu, art it.' 'Please, Sir, inform me still more,' said the son. 'Be it so, my child,' the father replied.